Determining the position and orientation of objects in free space has many applications. Specific applications include targeting systems used in conjunction with various aircraft, computer peripherals which utilize the position sensing apparatus to position a cursor, and many other pointing applications. Generally, either a single sensor or a plurality of sensors are attached to a device such as a helmet and the position of those sensors is determined relative to known reference points. In an aircraft targeting application, a plurality of sensors are placed in a pilot's helmet and the position of those sensors is determined. Knowing the position of the plurality of sensors, the orientation of the pilot's helmet can be calculated in relation to a reference axis of the aircraft. The orientation of the pilot's helmet can then be used to direct certain devices in the same general direction as the pilot's line of sight.
One prior approach to determining the position and orientation of an object is to utilize magnetic transmitters and receivers. Typically, a magnetic signal is transmitted of a known value from a known reference point. Receivers, or sensors, placed in free space sense the magnetic signal and the position of the receiver is calculated based on the magnitude and direction of the sensed magnetic field signal. The receivers used in this type of application are three axis magnetic sensors which can detect three orthogonal components of a magnetic field.
When these magnetic transmitters and sensors are utilized in an aircraft cockpit, adjustments must be made to account for the large amounts of metal in the area. The presence of a metal object in close proximity to the magnetic transmitter can seriously alter the strength and uniformity of a magnetic field. Since large amounts of metals exist on an aircraft, eddy currents in the metal produce magnetic fields which distort any transmitted magnetic fields at all points in the cockpit. Due to these eddy currents, signals received by the sensor are not true indications of position. These metal effects produce serious errors when trying to determine the position and orientation of sensors within a cockpit.
One solution to the problem of metal effects has been to map the area involved. More specifically, measurements are made in the cockpit wherein a known signal is generated and the sensor is positioned at a known position, resulting in a sensor signal. This sensor signal is then stored and the characteristics of the magnetic field within the entire cockpit are thus determined. Once the magnetic field characteristics are determined, the errors due to metal effects can thus be accounted for.
Another solution to the problem of metal effects is to utilize optical signals rather than magnetic signals. In this application an optical signal is generated from a known point within the cockpit and an optical sensor receives the optical signal. Several disadvantages are inherent in the use of optical signals. One disadvantage is the necessity to have a free path between the sensor and transmitter. Any object placed between the transmitter and the receiver will render the device inoperable and ineffective. Furthermore, the receivers will have a limited field of operation. For example, when an optical signal is placed directly behind the pilot's head and the optical sensor is placed directly on the back of the pilot's helmet, should the pilot turn his head too far the optical signal will not be received. Thus, there is a need for multiple optical sensors and multiple optical signal sources.